ABS molding compositions have been used for many years in large amounts as thermoplastic resins for producing molded items of all kinds. The range of properties of these resins can be varied to a wide extent.
ABS polymers with special combinations of properties are increasingly required due to the constantly increasing specifications for plastics materials and new areas of application.
A special combination of properties of this type concerns ABS polymers for the production of colored impact-resistant molded items, in particular in the field of application of housings and covering sheets. In this case, the smallest possible amounts of colorants are required to set the desired color and this can be achieved by a lower opacity for the polymer material and optionally by a paler color in the uncolored condition (lower yellowness index).
When using the technology of emulsion polymerization, attempts are usually made to achieve the desired properties by combining different grafted rubber components with a thermoplastic resin matrix.
Thus, for example, DE-A 24 20 357 and DE-A 24 20 358 describe the use of special grafted polymers, obtained by persulfate initiation, with defined values for, inter alia, rubber content, particle size, degree of grafting and styrene:acrylonitrile ratio, in the grafted rubber and in the styrene/acrylonitrile copolymer used, in order to produce improved values for strength, processability and surface gloss. Despite the relatively complicated mode of preparation of these molding compositions, optimum strength/flowability and strength/gloss relationships are not achieved and only inadequate inherent colors are produced.
Similar problems are demonstrated with the products prepared in accordance with EP-A 470 229, EP-A 473 400 and WO 91/13118, wherein impact-resistant, high-gloss, thermoplastic resins are obtained by combining a grafted polymer obtained by redox initiation having a low rubber content and a small particle diameter with a grafted polymer obtained by redox initiation having a high rubber content and a larger particle diameter. The flow characteristics and opacity of these molding compositions, however, do not comply with the constantly increasing specifications for these types of materials.
DE-A 41 13 326 describes thermoplastic molding compositions with two different grafted products, wherein the maximum rubber content of each grafted rubber is 30%. More precise data on the properties are not given in the document; the variability of the products and product properties must be very restricted, however, due to the low rubber content of the grafted polymers.
DE-A 196 49 255 discloses the preparation of ABS molding compositions with very high gloss indices while retaining good values for strength and processability, wherein a combination of a coarse-particled grafted polymer prepared by persulfate initiation and a fine-particled grafted rubber prepared by persulfate initiation is used with the maintenance of specific values for particle size distribution and gel content in the rubbers used.
The disadvantages of these products are, apart from the maintenance of numerous parameters required during preparation, are the not always adequate strength values and also the inadequate inherent color without adding colorants and too high opacity.
Thus, there is the object of providing compositions and thermoplastic molding compositions of the ABS type in which the disadvantages mentioned above do not occur, which have a combination of high strength and reduced opacity and in which the other properties are not adversely affected.
Even small reductions in opacity, which can be determined very accurately, lead to a marked reduction in the amounts of pigment required to color molding compositions.